If I can offer my analysis of the situation.
I don't actually think people are offended by the tweets of a singular Riot employee. They are angry at what Riot's response to those tweets implies about the values Riot is operating under.
First, there is the whole hypocrisy angle. Those tweets were clearly against Riot's terms of service and any player making such statements would, under the terms of service, have to face punishment for those statements. There is no evidence of Riot taking punitive actions, so it cannot be that the company actually cares about those values. And so why do they make their players adhere to them if they don't hold themselves accountable to the same measure.
This question of values is very important, because this hypocrisy suggests that Riot's value system is flawed.
To the minds of a player, the company should have 2 prime values. The first must obviously be financial. The company is beholden to make money for it's investors and employees or it cannot sustain it's self. This much is obvious, this much is acceptable. But the second, at least to the minds of players SHOULD be, to provide the best possible product.
You could make the argument, I suppose, that maintaining decorum in game increases the quality of the product. I suppose that's believably congruent with making a better game as the community is inextricably linked to the game. But there is no reasonable way that insulting and antagonizing 90% of your player base on social media fits in with that value. None at all.
So this brings into question Riot's core values. Because if their penultimate value ISN'T to make the best game possible, then we actually have identified an existential threat to the very game in question.
Does that makes sense?